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1. What is the main working principle of a centrifugal pump?

An electric motor drives the impeller to rotate at high speed, generating centrifugal force in the liquid. Due to this centrifugal force, the liquid is thrown into the side flow channel and discharged from the pump, or enters the next stage impeller. This reduces the pressure at the impeller inlet, creating a pressure difference with the pressure acting on the suction liquid. This pressure difference causes liquid to be drawn into the pump. As the centrifugal pump rotates continuously, liquid is constantly吸入 (sucked in) or discharged.

2. What are the functions of lubricating oil (grease)?

Lubrication and cooling, flushing, sealing, vibration damping, protection, and unloading.

3. What are the three stages of filtration that lubricating oil must go through before use?

First stage: Between the original lubricant drum and the fixed drum.

Second stage: Between the fixed oil drum and the oil can.

Third stage: Between the oil can and the lubrication point.

4. What is the "Five Fixed Points" of equipment lubrication?

Fixed Point:​ Lubricate at specified points.

Fixed Time:​ Lubricate at specified intervals and change oil periodically.

Fixed Quantity:​ Add oil according to the specified consumption amount.

Fixed Quality:​ Select the appropriate lubricant for different machine types and maintain oil quality standards.

Fixed Person:​ Assign a dedicated person responsible for each lubrication point.

5. What are the hazards of water in pump lubricating oil?

a. Water reduces the viscosity of lubricating oil, weakening the oil film strength and reducing lubrication effectiveness.

b. Water freezes below 0°C, seriously affecting the low-temperature fluidity of the lubricant.

c. Water accelerates oil oxidation and promotes corrosion of metals by low molecular organic acids.

d. Water increases the foaming tendency of lubricating oil.

e. Water causes metal components to rust.

6. What are the contents of pump maintenance?

a. Strictly implement the post responsibility system and equipment maintenance regulations.

b. Perform equipment lubrication according to the "Five Fixed Points" and "Three-Stage Filtration"; keep lubrication tools complete and clean.

c. Ensure maintenance tools, safety facilities, and fire-fighting equipment are complete, intact, and neatly placed.

7. What are the standard leakage rates for common shaft seals?

Packing Seal: Light oil < 20 drops/minute; Heavy oil < 10 drops/minute.

Mechanical Seal: Light oil < 10 drops/minute; Heavy oil < 5 drops/minute.

8. Why can't a centrifugal pump be started if the rotor cannot be turned by hand?

If the pump rotor cannot be turned, it indicates an internal fault. This could be the impeller jammed, the pump shaft severely bent, internal components seized due to corrosion, or excessive internal pressure. Forcibly starting a pump that cannot be turned may damage internal parts (e.g., broken or twisted shaft, shattered impeller), burn out the motor windings, or cause the motor to trip.

9. What is the function of seal flush oil?

a. Cool the sealing components.

b. Lubricate the friction pair.

c. Prevent damage from cavitation/suction loss.

10. Why do standby pumps require periodic turning?

a. Periodic turning has three main functions:

b. Prevent internal scaling from causing seizure.

c. Prevent pump shaft deformation.

d. Distribute lubricant to all lubrication points, preventing shaft rust and ensuring bearings are lubricated for immediate startup if needed.

11. What are the causes of abnormal heating in an operating centrifugal pump?

a. Heating indicates conversion of mechanical energy into heat. Common causes include:

b. Heating accompanied by noise, often due to damaged bearing cage.

c. Loose bearing retaining sleeve or end covers in the bearing housing causing friction.

d. Oversized bearing bore causing outer ring looseness.

e. Foreign object inside the pump casing.

f. Rotor vibration causing wear of the sealing ring.

g. Pump cavitation or overload.

h. Rotor imbalance.

i. Incorrect amount (too much/too little) or poor quality of lubricant.

12. What are the causes of centrifugal pump vibration?

a. Rotor imbalance.

b. Misalignment between pump and motor shaft; aged coupling spacer.

c. Excessive wear of bearings or sealing rings causing rotor eccentricity.

d. Pump cavitation or presence of gas.

e. Low suction pressure causing liquid to vaporize or near vaporization.

f. Increased axial thrust causing shaft displacement.

g. Improper lubrication of bearings and packing, excessive wear.

h. Worn or damaged bearings.

i. Partial blockage of impeller or vibration from external piping.

j. Too much or too little lubricant.

k. Insufficient base rigidity or loose bolts.

13. What are the standards for centrifugal pump vibration and bearing temperature?

Vibration Standard:

Speed < 1500 rpm: Vibration < 0.09 mm.

Speed 1500~3000 rpm: Vibration < 0.06 mm.

Bearing Temperature Standard:

Sliding bearings < 65°C.

Rolling element bearings < 70°C.

14. How much cooling water is appropriate during normal pump operation?

During normal operation, the cooling water flow should be adjusted so the outlet water temperature is about 40°C. At this point, touching the outlet water feels slightly warm, indicating optimal cooling. Especially for mechanical seal flush, avoid excessive flow in winter to prevent the seal rings from hardening due to low temperature and losing elasticity, leading to seal failure.

15. What are the symptoms of centrifugal pump cavitation?

When cavitation begins, the pump suddenly produces noise and vibration, accompanied by decreased pressure, reduced flow, and lower amperage. Severe cavitation causes strong pump vibration, pressure drop to zero, and no liquid discharge.

16. What are the causes of bearing housing overheating?

a. Misalignment between pump and motor shafts.

b. Incorrect amount (too much/too little) or degraded lubricant.

c. Broken or dislocated oil ring.

d. Increased axial thrust.

e. Insufficient or interrupted cooling water.

f. Worn-out bearings.

g. Very low flow, causing the pump to approach cavitation.

h. Vibration due to weak foundation, rotor imbalance, loose bolts, etc.

17. Why must pumps be protected from freezing in winter?

Water expands below 0°C. If water remains trapped inside the pump casing, the expansion force at low temperatures can crack the pump body. Anti-freezing methods include:

a. Drain all water from idle pumps.

b. Maintain a continuous trickle flow of cooling water.

c. Insulate the pump or use steam/tracing.

d. Keep the suction and discharge lines of standby pumps open.

18. How to handle a frozen pump?

a. Never use steam directly on a frozen pump, as uneven thermal expansion can crack the casing.

b. First, apply cold water. Then, once the rotor can be turned, use steam or hot water carefully.

19. Why is the pump casing cooled?

During operation, heat is generated by fluid friction within the pump and mechanical friction in components like bearings and seals. Heat is also conducted from the high-temperature medium. Cooling aims to reduce the temperature of the pump casing, baseplate, bearing housing, and seal area to prevent deformation, aging, or damage due to high temperature.

20. Under what conditions should an electric motor be stopped urgently?

a. When personal safety is endangered.

b. If the motor smokes, emits a burning smell, or catches fire.

c. If severe vibration or axial movement occurs.

d. If the motor body or bearing temperature reaches the maximum allowable limit.

e. If the motor speed drops abnormally accompanied by unusual noises.

21. Interpret the model of a domestic centrifugal oil pump. For example, what does 100YⅡ--100×2A represent?

100: Pump suction diameter (mm).

Y: Single-stage centrifugal pump.

Ⅱ: Pump material code (second type carbon steel, not corrosion-resistant, temp. 200-400°C).

100×2A: Head per stage x number of stages; A indicates the first cut of the impeller diameter.

22. How to identify cavitation phenomenon?

a. Loud knocking or cracking noises inside the pump.

b. Unstable discharge pressure; severely impaired flow or no flow.

c. Pump vibration.

d. Increase in medium temperature.